1. Field of the Invention
The present invention relates to an MRAM (Magnetic Random Access Memory) device, and in particular, to a write test mode in an MRAM device which uses magnetic memory cells each composed of an element that stores “1”/“0” information on the basis of the tunneling magnetoresistive effect.
2. Description of the Related Art
In recent years, a large number of memories have been proposed which store information on the basis of new principles. One of them is a non-volatile and high-speed MRAM device having magnetic memory cells arranged in a matrix and each composed of a magnetic tunnel junction (hereinafter referred to as an “MTJ”) element that stores “1”/“0” information using the tunneling magnetoresistive effect. Such an MRAM device is disclosed in, for example, Roy Scheuerlein et. al. “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest pp. 128 to pp. 129.
In the conventional MRAM device, chips or cell arrays may have different MTJ element switching characteristics or different asteroid curves owing to manufacturing variations. Owing to these various switching characteristics, writes may be disabled if the asteroid curves for the MTJ elements are distorted, i.e. if the asteroid curves are asymmetric with respect to the X and Y axes by the positional relationship between write lines and the MTJ elements, the asymmetric MTJ plane shape with respect to the X and Y axes, the each thickness variation of the MTJ component materials, etc.
Furthermore, with the conventional MRAM device, cells that have barely passed a write test may reject writes during actual operations. Desirably, the cells that barely pass a write test can be detected during the test.